The invention relates to II-VI semiconductor diode lasers, and in particular to II-VI semiconductor diode lasers employing a strained layer to cause strain-induced index guiding and/or strain-enhanced gain.
Presently, most commercially-available semiconductor diode lasers are fabricated of III-V compound semiconductors and their alloys. Such semiconductor diode lasers, which emit light in the infrared and red portions of the spectrum, are used inter alia as components in read and/or write heads of information processing equipment such as laser printers, bar code readers, and read and/or write devices for optical registration carriers such as CD and CD-ROM discs, and as transmitters in systems for optical glass fiber communication. In the latter systems, the semiconductor diode laser may also be used as a light amplifier, and in this application the term "laser" is meant to include such light amplifiers.
However, there are many applications for which the wavelength of light generated by infrared and red diode lasers is not suitable. Other types of semiconductor diode lasers, such as II-VI semiconductor diode lasers which emit light at shorter wavelengths, for example the green and blue portions of the spectrum, would accordingly be desirable. These semiconductor diode lasers would also increase the performance and capabilities of many systems which currently use infrared and red semiconductor diode lasers.
Semiconductor diode lasers consist of semiconductor layers, and require light confinement in two directions in order to operate efficiently. The first direction in which confinement is needed is the vertical direction perpendicular to the semiconductor layers. This confinement is realized by cladding layers spaced above and below the active layer in the laser. Confinement in a second direction, i.e. in the lateral direction parallel to the layers, is also necessary in semiconductor diode lasers. In known II-VI semiconductor diode lasers, this confinement is realized by laterally confining the injection current that gives rise to optical gain. These structures are called "gain-guided" structures. However, this type of confinement is not very efficient. For enhanced lateral confinement, an etching process can be performed on the layer structure in order to form, for example, a ridge structure. The effective refractive index under the ridge is then greater than the effective refractive index alongside it. This difference in refractive indices induces additional lateral confinement. The structures employing this guiding mechanism are called "index-guided" structures. In most cases, the manufacture of these structures require some form of etching of semiconductor material, which is generally not an easy step.
Both of the laser structures described above require an injection current to produce laser light. The minimum current that is needed is called the threshold current. Since a high current will use more power, produce more heat, and will result in a shorter life for the semiconductor diode laser, it is desirable to keep the threshold current as low as possible consistent with reliable operation.